whats the flow of water light when you flush the matrix?? just a trickle?
it might be worth seeing if you can stick your hand under the dash (or in the air vent chamber) and feel the matrix its self. might be easier if you feel the metal pipes going to and from the matrix (they stick through the bulk head into the engine bay) one might be hot one might be cold.
try shutting the matrix vlave (turning the dial to cold) starting the engine and letting it warm up) then feeling the pipes get someone to open up the valve and both the inlet and outlet pipes on the matrix should become hot/warm
how do you fill the system up with coolant once you drained it? very very slowly i hope (with the heater turned to hot) otherwise you will get air locks.
i had a similar problem with my old 216, after a head gasket change i could not get my heater to blow hot again, i tried draining and refilling, bleeding, flushing it all seemed ok but at best all i could get was luke warm air. i never got it fixed but i sold the car now.